it might help to know at least what continent you are on..... I don't think there is a good global guide
Added:  whoops checking your other posts, seems you are in Sydney, sorry, not aware of anything for that area.  Dandelions etc are generally easy to find.  Are you after local native wild stuff?  The staff at the nursery at Muru Mittigar Aboriginal Centre are usually very helpful. http://www.murumittigar.com.au/old/nursery It's a bit past Penrith on Old Castlereagh Rd, the first bit of it, very easy to miss the signs on the righthand side.

here is a link to bush foods
www.teachers.ash.org.au/bushtucker/index.html

here is alink to wild Australian plants